The chemistry of polyurethanes and polyureas is extensive and well developed. Typically, polyurethanes and polyureas are made by a process in which a polyisocyanate is reacted with a molecule having at least two hydrogen atoms reactive with the polyisocyanate, such as a polyol or polyamine. The resulting polymer can be further reacted with a chain extender, such as a diol or diamine, for example. The polyol or polyamine can be a polyester, polyether, or polycarbonate polyol or polyamine, for example.
Polyurethanes and polyureas can be tailored to produce a range of products from soft and flexible to hard and rigid. They can be extruded, injection molded, compression molded, and solution spun, for example. Thus, polyurethanes and polyureas, particularly polyurethanes, are important biomedical polymers, and are used in implantable devices such as artificial hearts, cardiovascular catheters, pacemaker lead insulation, etc.
Commercially available polyurethanes used for implantable applications include BIOMER segmented polyurethanes, manufactured by Ethicon, Inc., of Sommerville, N.J., PELLETHANE segmented polyurethanes, sold by Dow Chemical, Midland, Mich., and TECOFLEX segmented polyurethanes sold by Thermedics, Inc., Woburn, Mass. These polyurethanes and others are described in the article "Biomedical Uses of Polyurethanes," by Coury et al., in Advances in Urethane Science and Technology, 2, 130-168, edited by Kurt C. Frisch and Daniel Klempner, Technomic Publishing Co., Lancaster, Pa. (1984). Typically, polyether polyurethanes exhibit more biostability than polyester polyurethanes, and are therefore generally preferred biopolymers.
Polyether polyurethane elastomers, such as PELLETHANE 2363-80A (P80A) and 2363-55D (P55D), which are believed to be prepared from polytetramethylene ether glycol (PTMEG) and methylene bis(diisocyanatobenzene) (MDI) extended with butane diol (BDO), are widely used for implantable cardiac pacing leads. Pacing leads are insulated wires with electrodes that carry stimuli to tissues and biologic signals back to implanted pulse generators. The use of polyether polyurethane elastomers as insulation on such leads has provided significant advantage over silicone rubber, primarily because of the higher tensile strength and elastic modulus of the polyurethanes.
There is some problem, however, with stress cracking of polyether polyurethane insulation, which can cause failure. Polyether polyurethanes are susceptible to oxidation in the body, particularly in areas that are under stress. When oxidized, polyether polyurethane elastomers lose strength and form cracks, which eventually breach the insulation. This, thereby, allows bodily fluids to enter the lead and form a short between the lead wire and the implantable pulse generator (IPG). It is believed that the ether linkages degrade, perhaps due to metal ion catalyzed oxidative attack at stress points in the material.
One approach to solving this problem has been to coat the conductive wire of the lead. Another approach has been to add an antioxidant to the polyurethane. Yet another approach has been to develop new polyurethanes that are more resistant to oxidative attack. Such polyurethanes include only segments that are resistant to metal induced oxidation, such as hydrocarbon- and carbonate-containing segments. For example, polyurethanes that are substantially free of ether and ester linkages have been developed. This includes the segmented aliphatic polyurethanes of U.S. Pat. No. 4,873,308 (Coury et al.). Although such materials produce more stable implantable devices than polyether polyurethanes, there is still a need for biostable polymers, particularly polyurethanes suitable for use as insulation on pacing leads.